This invention relates, in general, to molding processes, and more particularly, to a molding process in which a thixotropic compound is used for encapsulating semiconductor devices.
In the past, semiconductor devices were encapsulated using a transfer molded filled epoxy compound or solid thermoset. This solid thermoset is a solid at room temperature and is input to the process in the form of pellets or tablets made by compacting a coarse granule. The granules must be stored in a cold storage before use. In addition, the pellets must be metered to match different sizes of molding apparatus. The volumetric controls required using this epoxy compound are not only time consuming, but add no value to the process, thus are very disadvantageous.
The molding process of the prior art is carried out as follows: a piston or ram, acting within a cylinder, called a pot, plasticizes pellets of solid thermoset, forcing the solid thermoset into distribution runners of a mold. The compound must also be preheated. The solid thermoset flows in the distribution runners where it is fluidized by shear flow through subrunners and gates to mold cavities. In the cavities, the now liquid thermoset is warmed to mold temperature where it cures to form a rigid solid. One disadvantage of this process is that the transfer cylinder or pot and the transfer ram operate at the high mold temperatures. Thus, a cured cull remains in the pot and sprue remains in the distribution runners and subrunners, which constitutes a substantial waste of the solid thermoset. A specific amount of solid thermoset must be placed into the pot so that a cull remains, otherwise, air may enter the mold cavity. Because the solid thermoset is exposed to the environment before injection into the mold, particulates and water may contaminate the solid thermoset, diminishing the quality of the molded compound.
By now it should be appreciated that it would be advantageous to provide an improved molding process.
Accordingly, it is an object of the present invention to provide an improved molding process utilizing a thixotropic liquid thermoset compound.
Another object of the present invention is to provide an improved molding process for encapsulating semiconductor devices.
A further object of the present invention is to provide a molding process that reduces the amount of waste.
An additional object of the present invention is to provide a molding process that increases the efficiency of transfer molding.
Yet another object of the present invention is to provide a molding apparatus that improves product quality.
Still another object of the present invention is to provide a molding process that does not require volumetric controls.